Method of fabricating shaped crystals by overhead-pressure liquid injection

ABSTRACT

A shaper (2) is arranged within a shaping vessel (1). A raw material for a crystal is inserted into the shaping vessel (1) and a crystal melt (5) is formed by setting it in a predetermined atmosphere and heating it. A mechanical force F1 is applied to the crystal melt (5), which is present on the upper surface of the shaper (2), by a pressuring member (4) from above. The crystal melt (5) has nowhere to escape but a gap (3) formed by the shaper(2), so it is injected into that gap (3) as shown by the arrow. This method of fabricating a shaped crystal is suitable for fabricating a monocrystal or multicrystal semiconductor from a material such as silicon, germanium, or bismuth telluride.

TECHNICAL FIELD

The present invention relates to a method of fabricating a semiconductormonocrystal or multicrystal of a material such as silicon, germanium, orbismuth telluride, and, in particular, to a method of fabricating asemiconductor crystal that is formed to have a predetermined shape.

BACKGROUND OF ART

A method of fabricating a large number of cube-shaped monocrystal pieceswith sides on the order of 1 to 3 mm is known in the prior art as amethod of fabricating pieces of a semiconductor crystal, such as amonocrystal or multicrystal semiconductor of a bismuth/telluriumcompound; which involves slicing an ingot of monocrystal or multicrystalgrown in an a-axial direction by a downward-drawing method to formcircular plates, then dicing these circular plates.

Other methods, such as Edge-defined Film-fed Growth (EFG) and crosscutmethods, are also known in the prior art as methods of fabricatingsheets of semiconductor crystal such as silicon ribbon crystals.

However, the above method of fabricating monocrystal or multicrystalsemiconductor pieces of a bismuth/tellurium compound necessitates acutting process of one slicing step and two dicing steps, so it isexpensive.

Furthermore, the silicon melt solidifies at a speed of ten to severalhundred mm/minute with the above EFG and crosscut methods, so the grainsize of the completed crystals is small and thermal stresses stillremain therein, and thus quality is poor.

The present invention was devised in the light of the above problems andhas as an objective thereof the provision of an inexpensive method offabricating shaped crystals of a needle shape (diameter: approximately0.5 mm to 3 mm) or sheet shape (thickness: approximately 0.5 mm) from ahigh-quality bismuth/tellurium compound or other semiconductor.

DISCLOSURE OF INVENTION

This invention concerns a method of fabricating a shaped crystal byoverhead-pressure liquid injection, comprising a step of heating a rawcrystal material within a shaping vessel set in a predeterminedatmosphere, to form a melt; a step of pressuring the melt from overhead,to inject the same into a shaper disposed within the shaping vessel; anda step of cooling the melt injected into the shaper, to form a crystal.

The melt could be injected inward from above or below the shaper. Thepressuring during this time could be performed by a mechanical force orby the use of a gas pressure that is set by means for setting theatmosphere of the shaping vessel.

If the shaping vessel is formed to have a box shape and a plurality ofconical holes are provided in the base of the interior thereof forcontrolling the orientation of crystals, it is possible to controla-axis orientation.

If the shaper is configured of a plurality of plate members, each havinggrooves of a predetermined shape in the vertical direction and beinglinked together in such a manner that the grooves are opposing, a largenumber of needle crystals can be fabricated simultaneously.

If the shaper is configured of a linked series of a plurality of platemembers formed to have steps of predetermined dimensions along left andright edges thereof, a plurality of sheet crystals can be fabricatedsimultaneously.

Since the present invention can thus be used to fabricate a number ofshaped crystals simultaneously, fabrication costs are reduced. The speedof growth of the crystals can also be slowed so that high-qualitycrystals are obtained.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a first example of the crystal melt injection method inaccordance with this invention;

FIG. 2 shows a second example of the crystal melt injection method inaccordance with this invention;

FIG. 3 shows a third example of the crystal melt injection method inaccordance with this invention;

FIG. 4 shows essential components of a device for fabricating needlemonocrystals of a thermoelectric compound semiconductor;

FIG. 5 is a perspective view of the shaping vessel

FIG. 6 is a vertical cross-sectional view of the shaping vessel;

FIG. 7 is a plan view of the shaping vessel;

FIG. 8 shows the dimensions of a conical hole within the shaping vessel;

FIG. 9 is a perspective view of a shaper;

FIG. 10 is a plan view of the shaper shown in FIG. 9;

FIG. 11 is a perspective view of a shaper;

FIG. 12 is a plan view of the shaper shown in FIG. 11;

FIGS. 13A to 13D show the process of fabricating needle monocrystals;

FIG. 14 shows essential components of a device for fabricating sheetmonocrystals of germanium;

FIG. 15 shows a detail of the interior of a shaping vessel when a rawcrystal material of germanium is in a molten state;

FIG. 16 is a perspective view of the shaper;

FIG. 17 is another perspective view of the shaper;

FIG. 18 is a view showing the shaper as seen from a front surfacethereof, in a state in which raw crystal material is present in the basewithin the shaping vessel;

FIG. 19 is a view showing the shaper as seen from a side surfacethereof, in a state in which raw crystal material is present in the basewithin the shaping vessel;

FIG. 20 shows the raw crystal material in a molten state;

FIG. 21 shows a state in which the melt has been injected into the gapswithin the shaper;

FIG. 22 shows a state in which the melt is solidifying from a lower leftside of the shaper;

FIG. 23 shows a state in which the temperature of the crystal hasfallen;

FIG. 24 shows the shaper being removed from the shaping vessel;

FIG. 25 shows a sheet crystal being removed from the shaper; and

FIG. 26 shows essential components of a third embodiment of thisinvention.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of this invention will be described below with reference tothe accompanying drawings.

(1) Crystal Melt Injection Methods

First Method

A first example of the crystal melt injection method in accordance withthis invention is shown in FIG. 1. This figure is a section taken in thevertical direction. With this method, a shaper 2 is disposed within ashaping vessel 1. A crystal melt 5, which is present on an upper surfaceof the shaper 2, is pressed from above by a pressuring member 4. Duringthis time, a mechanical force F1 is applied to the pressuring member 4from above. When the crystal melt 5 is pressed by a lower edge of thepressuring member 4, it has nowhere to escape but a gap 3 formed in theshaper 2, so it is injected into the gap 3 as shown by the arrow in FIG.1.

Note that details of the materials and configuration of the shapingvessel 1 and the shaper 2, as well as the conditions (such astemperature and atmosphere) necessary during the injection of thecrystal melt and the processing before and after that injection, will begiven later in the description of embodiments of the method offabricating a shaped crystal.

Second Method

A second example of the crystal melt injection method in accordance withthis invention is shown in FIG. 2. In this case, portions that areidentical to those shown in FIG. 1 are denoted by the same referencenumbers as those in FIG. 1. In this method, the shaper 2, which ispresent on the upper surface of the crystal melt 5, is pressed fromabove by the pressuring member 4. During this time, the mechanical forceF1 is applied to the pressuring member 4 from above. When the shaper 2is pressed by a lower edge of the pressuring member 4, it moves downwardinto the vessel 1. This time, the crystal melt 5 has nowhere to escapebut the gap 3 formed by the shaper 2, so it is injected into the gap 3as shown by the arrow in FIG. 2.

Third Method

A third example of the crystal melt injection method in accordance withthis invention is shown in FIG. 3. In this case, portions that areidentical to those shown in FIG. 1 are denoted by the same referencenumbers as those in FIG. 1. In this method, the crystal melt 5, which ispresent on an upper surface of the shaper 2, is pressed from above by apressuring member 6. During this time, a gas pressure F2 is applied tothe pressuring member 6. When the crystal melt 5 is pressed by a loweredge of the pressuring member 6, it has nowhere to escape but the gap 3formed by the shaper 2, so it is injected into the gap 3 as shown by thearrow in FIG. 3.

With this method, the raw material for the crystal is first placedbetween the shaper 2 and the pressuring member 6, and it is also placedon top of the pressuring member 6. Next, the interior of a chamber 8 isevacuated by the operation of a vacuum pump 11. The raw material of thecrystal is then melted by a heating means (not shown in the figure)provided around the shaping vessel 1. A valve 10 is opened to introduceargon gas, which is within a gas cylinder 9, into the chamber 8. Sincethe lower surface of the pressuring member 6 is in a vacuum, the gaspressure of the argon gas moves it to below the pressuring member 6.During this time, a part 7 of the molten raw crystal material acts toblock a gap between the pressuring member 6 and an inner wall of theshaping vessel 1, so that there are no problems, even with componentsthat have a bad machining accuracy and a lower level of airtightness.

Note that the evacuation of the atmosphere of the shaping vessel 1 byuse of a vacuum pump and the melting of the raw crystal material by theheating means are the same as in the previously described first andsecond methods

(2) Methods of Fabricating Shaped Crystals

First Embodiment

The present embodiment is designed to inject a melt of a thermoelectricsemiconductor crystal such as bismuth telluride from below into theinterior of a shaper, to fabricate a large number of needle monocrystalssimultaneously.

Essential components of the device for fabricating needle monocrystalsof a thermoelectric compound semiconductor are shown in FIG. 4. Ashaping vessel 22 is provided for accommodating the raw material for thecrystal in the interior of a quartz tube 21. A shaper 23 for molding themolten raw crystal material into needle shapes is provided in theinterior of the shaping vessel 22. The shaper 23 moves downward into theinterior of the shaping vessel 22, pressed from above by a pressuringmember 25. During this time, the melt in the base portion of the shapingvessel 22 is injected into elongated holes 24 formed in the shaper 23.In other words, this is basically equivalent to the situation shown inFIG. 2.

The temperatures and temperature gradient in the vertical directionwithin the quartz tube 21 are controlled by a heater 26. Note that,although this is omitted from the figures herein, portions of the heaterother than those shown in FIG. 4 are provided in a metal vessel such asthe chamber 8 of FIG. 3.

An example of the configuration of the shaping vessel 22 is shown inFIGS. 5 to 8. In this case, FIG. 5 is a perspective view, FIG. 6 is avertical cross-sectional view, FIG. 7 is a plan view, and FIG. 8 showsthe dimensions of a conical hole. The shaping vessel 22 is made ofcarbon or a ceramic such as boron nitride and is formed to have a boxshape, as shown in FIG. 5 or a circular cylindrical shape. A largenumber (25, in these figures) of a diameter of 5 mm and a depth of 10 mmare provided in a surface of the base of the interior of the shapingvessel 22, as shown in FIGS. 6 to 8.

Examples of the configuration of the shaper 23 are shown in FIGS. 9 to12. In this case, FIGS. 9 and 11 are perspective views of shapers andFIGS. 10 and 12 are plan views corresponding to FIGS. 9 and 11,respectively. In the example shown in FIGS. 9 and 10, three curvedgrooves 231 of a semi-circular section are cut in the vertical directionof a plate member, and two plate members are superimposed so that thecurved grooves 231 are opposing (to make the relationship between thetwo members easier to understand, they are separated slightly in FIG.10). A plurality of these superimposed members are themselvessuperimposed. This configuration enables easy removal of needlemonocrystals formed therein, by mutual separation of the plate members.

In FIG. 10, curved grooves could be cut into both sides of each platemember, apart from the two edges thereof. The shape of the grooves couldalso be triangular or square. FIGS. 11 and 12 show a configuration inwhich angular grooves 232 of a triangular section-shaped are cut. Inessentials, the shape and dimensions of the grooves can be determined bythe shape and dimensions of the needle crystals to be fabricated.

Note that, although three grooves are shown cut into each plate member23 in this case, it should be obvious that five grooves could be cuttherein as necessary, to correspond to FIGS. 6 and 7. It is similarlyobvious that five pairs of plate members (i.e. ten plates) could beinserted into the shaping vessel 22.

The crystal axis that brings out the characteristics of abismuth/tellurium compound thermoelectric material the best is that inthe a-axial direction. It is therefore preferable to grow crystal insuch a manner that the a-axis is orientated in the longitudinaldirection of the needle crystals. That is why a large number of theconical holes 221 is provided in the lower surface within the shapingvessel 22 of this embodiment, as previously described. It has beenverified that, if the thus-configured shaping vessel is used, the actionof the conical holes 221 ensures that virtually all of the crystal formsin an orientation close to the a-axis in the direction of the grooves231 or 232.

The reason why the conical holes 221 induce this a-axis orientation isthought to be as described below. Crystallization of the bismuthtelluride melt in the base portion of the shaping vessel 22 starts fromthe points of the conical holes 221. The start point of thiscrystallization is the sharp tip of each cone, and, in this case,seed-crystallization that is self-orientated along the a-axis of thetiny crystal occurs, so the growth moves gradually toward the upper partof that conical hole 221.

A method of promoting monocrystallization by sharpening a tip portionfor crystallization into a conical shape has long been known among themethods of growing crystals by using a vessel, but there are no knownexamples in which a large quantity of needle monocrystals arecrystallized from a large number of seed crystals, as in thisembodiment.

Since the shaper 23 is in direct contact with the bismuth telluridemelt, mutual wetting and reactions therebetween would cause the needlemonocrystals to attach to the shaper after the crystallization, makingremoval impossible. It is therefore important to select the material ofthe shaper 23 and the method of use thereof. In this embodiment, agraphite object ET-10P, made by Ibiden Co., Ltd., is used as thematerial of the shaper 23. Since the surfaces of the curved grooves 231or angular grooves 232 of the shaper 23 are subjected to a surfacetreatment by coating with a solid lubricating agent, it has beenconfirmed that there is absolutely no adhesion between the grooveportions and the needle crystals and the needle crystals can be removedin a simple manner from the shaper 23. The solid lubricating agent usedin this case is a boron nitride paint (trade name: Whitey Paint) made byAudek.

The process of fabricating needle monocrystals will next be describedwith reference to FIGS. 13a to 13d. Note that the conical holes 221provided in the interior of the shaping vessel 22 are omitted from thisfigure, for convenience.

First of all, as shown in FIG. 13a, a raw material 27 for fabricatingthe thermoelectric compound semiconductor monocrystal is placed in thebase portion of the shaping vessel 22, a vacuum pump (not shown in thefigure) and the heater 26 are operated to remove any impurities from thesurfaces of the raw material and any water vapor or oxygen from withinthe shaping vessel 22, then the raw material 27 is melted while an inertatmosphere that will prevent chemical bonding between the crystal andthe shaper 23 is created within the shaping vessel 22.

When the raw material 27 has melted, an external force F, such as gaspressure, is applied to the pressuring member 25 to move the resultantmelt 28 in the direction of the shaper 23, as shown in FIG. 13b. Duringthis time, the melt 28 is injected into the elongated holes 24 formed bythe grooves of the shaper 23.

The shaper 23 is moved as far as the base of the shaping vessel 22, asshown in FIG. 13c, and, once the melt 28 has completely filled theelongated holes 24 of the shaper 23, the quartz tube 21 is graduallymoved downward, as shown in FIG. 13d, to cause the melt 28 filling theelongated holes 24 of the shaper 23 to solidify from below. During thistime, the temperature gradient in the vertical direction within thequartz tube 21 (which is on the order of 20° to 40° C./cm for abismuth/tellurium compound, for example) controls the crystal axisdirection to grow in the a-axis direction, due to the provision ofconical holes in the lower surface. Note that, instead of lowering thequartz tube 21 in this case, the temperature of the heater 26 couldequally well be controlled to cool and solidify the crystals.

Once the melt 28 has solidified and the crystals have formed, the shaper23 is removed from the shaping vessel 22 and a large number of needlemonocrystals are removed from the elongated holes 24. In this case,needle multicrystals could also be fabricated by cooling the melt 28quickly.

When the thus fabricated needle crystals of bismuth/tellurium compoundare machined into pieces, a single cutting step alone is sufficient,enabling a huge reduction in the number of steps in comparison with theprior-art technique that requires one slicing step and two cutting steps4a total of three steps.

Second Embodiment

In this embodiment, the crystal melt of germanium is injected into theshaper from above, to fabricate a large number of sheet crystalssimultaneously.

A gas cylinder 32 filled with argon gas is connected to a quartz tube31, as shown in FIG. 14, and this argon gas can be introduced into thequartz tube 31 by opening a valve 33. A vacuum pump 34 is also connectedto the quartz tube 31 and the interior of the quartz tube 31 can beevacuated by the operation thereof. A heater 35 is provided around thequartz tube 31 (in this figure, it is only shown below it), and theinterior of the quartz tube 31 can be held at a predeterminedtemperature thereby. A stand 36 is disposed on a base portion within thequartz tube 31 and a shaping vessel 37 containing a raw crystal materialis disposed on this stand 36. A shaper 38 for shaping the molten rawcrystal material into sheets is provided within the shaping vessel 37.

The interior of the shaping vessel 37 is shown in detail in FIG. 15, ina state in which a raw germanium crystal material is molten. The shaper38 is placed in a base portion of the interior of the shaping vessel 37.A melt 41 of the germanium crystal is above the shaper 38. A pressuringplate 40 is disposed above the melt 41 of the germanium crystal, amaterial 47 such as lithium fluoride or potassium chloride is disposedthereabove, and a pressuring member 39 is disposed above that. In otherwords, it should be obvious that this configuration is basicallyequivalent to that of FIG. 1, except that the melt is disposed about theshaper as shown in FIG. 2.

The shaping vessel 37 is formed in a box shape of carbon or a ceramicsuch as boron nitride, in the same manner as in the first embodiment.The shaper 38 is of graphite, also in the same manner as in the firstembodiment. The melting point of the lithium fluoride 47 is 842° C.,less than the melting point of germanium which is 937° C. Therefore,when the germanium crystal melts, the lithium fluoride 47 also melts andacts to seal the gap between the inner wall of the shaping vessel 37 andthe pressuring plate 40. It is therefore possible to use a shapingvessel 37 and pressuring plate 40 with a poor machining accuracy and lowlevel of airtightness.

Perspective views of the shaper 38 are shown in FIGS. 16 and 17. Asshown in these figures, the shaper 38 consists of plates havingdimensions of a width W, a height H, and a depth D, each with a step ofwidth Wd, height H, and depth d on each of the left and right edgesthereof. A large number (e.g. 150) of the shapers shown in FIG. 16 aredisposed in a linked series within the shaping vessel 37 shown in FIG.15. Thin gaps of a width (W-2Wd), a height H, and a depth (D-d) are thusformed between neighboring shapers. The state of three linked shapers isshown in FIG. 17. The melt 41 shown in FIG. 15 is injected into thesegaps, and is cooled to form sheet crystals. Therefore the variousdimensions of the shaper 38 can be determined in accordance with thedimensions and thickness of the sheet crystals to be fabricated.

The description now turns to the process of fabricating germaniummonocrystal sheets.

First of all, a raw material 42 for the fabrication of germaniumcrystals is placed in a base portion of the shaping vessel 37, as shownin FIGS. 18 and 19, the vacuum pump 34 and heater 35 shown in FIG. 14are operated to remove any water vapor or oxygen from within the shapingvessel 37, then the raw material 42 is melted while an inert atmospherethat will prevent chemical bonding between the crystal and the shaper 38is created within the shaping vessel 37. In this case, FIG. 18 shows theshaper 38 as seen from the direction of the front surface thereof andFIG. 19 shows the shaper 38 as seen from the direction of a side surfacethereof.

When the raw material has melted to form a melt 43, as shown in FIG. 20,a force F is applied to the pressuring member 39 of FIG. 14 to move itin the direction of the shaper 38. As a result, the melt 43 is injectedinto the gaps formed within the shaper 38. The melt 43 is graduallysolidified from below by gradually lowering the temperature of theheater 35. A state in which a crystal 44 is formed by solidificationfrom the lower left-hand side is shown in FIG. 22. Similarly, a state inwhich a crystal 45 is formed by solidification of the entire melt isshown in FIG. 23.

When the entire melt has solidified and cooled to room temperature, asshown in FIG. 24, the shaper 38 is removed from the shaping vessel 38and plate crystals 46 are removed from the shaper 38, as shown in FIG.25. A large number of sheet crystals can be obtained simultaneouslybecause a large number (e.g. 150) of shapers are disposed within theshaping vessel 37, as previously mentioned.

Third Embodiment

Essential components of a third embodiment of this invention are shownin FIG. 26. This embodiment expands on the second embodiment to providea configuration that can be applied to mass production. Portions thatare equivalent to those of the second embodiment are denoted by the samereference numbers.

In this embodiment, the configuration is such that a large number (7, inthis figure) of shaping vessels 37 move along graphite rails 50 at a lowspeed (in this case: 300 mm/H) from right to left. To enable this, alinear shaft is driven by a motor, not shown in the figure, so that theshaping vessel at the right-hand end is pushed at a low speed by thislinear shaft. Although this is not shown in the figure either, amechanism is provided to load the shaping vessels 37 one at a time ontoa right-hand end portion of the rails and remove these shaping vessels37 one at a time from a left-hand end portion of the rails 50. Theheater 35 is disposed around the path along which the shaping vessels 37move (but is shown only thereabove in the figure).

Raw crystal material 42 is contained in the base portion of the shapingvessel 37 at the right-hand end. The raw crystal material 42 issubjected to strong heating from the heater 35 as it moves along therails 50 from right to left and, when it passes its melting point, itbecomes the melt 43. The shaper 38 is pressed downward by the pressuringmember 39 at substantially the center of the movement path thereof, sothat the melt 43 is injected into the gaps formed in the shaper 38. Themelt 43 injected into the gaps of the shaper 38 are subjected to weakerheating as it moves away from the center of the movement path, andgradually solidifies from the lower left-hand side to form the crystal44. A low-temperature crystal 45 is further formed as the movementcontinues to the left. If it is assumed that the width of the shapingvessels 37 in FIG. 26 is 150 mm and the operation speed is 300 mm/H, andif 150 shapers 38 are provided within each shaping vessel 37, thepressuring member 39 presses the shaper 38 at a rate of twice per hour.In this case, the number of sheet crystals that can be fabricated in oneday is: 150×2×20=6000 crystals.

It should be noted that, although the above described embodiments weredescribed with reference to bismuth/tellurium compound semiconductorcrystals and germanium crystals, the present invention can also beapplied to the fabrication of semiconductor crystals such as siliconcrystals and gallium/antimony crystals, fluoride crystals, oxidecrystals, or metal crystals. Similarly, the description of the aboveembodiment related to the fabrication of needle or sheet crystals, thepresent invention can be applied to the fabrication of other shapes ofcrystal, by changing the form of the shaper.

INDUSTRIAL APPLICABILITY

The method of fabricating a shaped crystal in accordance with thisinvention is useful for fabricating various kinds of crystals such asbismuth/tellurium compound thermoelectric semiconductor crystals,silicon crystals, or germanium crystals, and is particularly suitablefor the fabrication of a large number of highly functional monocrystalsin batches.

I claim:
 1. A method of fabricating a shaped crystal, comprising:a stepof heating a raw crystal material within a shaping vessel to form amelt; a step of pressuring the melt from overhead, so as to inject themelt into a shaper disposed within said shaping vessel; and a step ofcooling melt injected into said shaper, to form a crystal.
 2. The methodof fabricating a shaped crystal by overhead-pressure liquid injection ofclaim 1, wherein the melt is injected into the shaper from above theshaper.
 3. The method of fabricating a shaped crystal byoverhead-pressure liquid injection of claim 1, wherein the melt isinjected into the shaper from below the shaper.
 4. The method offabricating a shaped crystal by overhead-pressure liquid injection ofclaim 1, further including means for pressuring said melt by using a setgas pressure.
 5. The method of fabricating a shaped crystal byoverhead-pressure liquid injection of claim 1, wherein the shapingvessel has a box shape and defines a plurality of conical holes in thebase of the interior thereof for controlling the orientation ofcrystals.
 6. The method of fabricating a shaped crystal byoverhead-pressure liquid injection of claim 5, wherein the shaper isconfigured of a plurality of plate members, each having grooves in thevertical direction and being linked together in such a manner that saidgrooves are opposing.
 7. The method of fabricating a shaped crystal byoverhead-pressure liquid injection of claim 1, wherein the shaper isconfigured of a linked series of a plurality of plate members formed tohave steps along left and right edges thereof.
 8. The method offabricating a shaped crystal by overhead-pressure liquid injection ofclaim 1, wherein the raw crystal material is heated in a vacuum withinthe shaping vessel.
 9. The method of fabricating a shaped crystal byoverhead-pressure liquid injection of claim 1, wherein the raw crystalmaterial is heated in an inert atmosphere within the shaping vessel.